It is well known to use an eye diagram to monitor the suitability of a transmission path (which term is used herein to designate both a broadcast medium, such as air, and a transmission line) between a transmission location and a remote receiving location for transmission of digital data. The eye diagram is conventionally generated by impressing a sinusoidal signal on the transmission path at the transmission location, and displaying the signal received at the receiving location on a dual-trace oscilloscope-type cathode ray tube (CRT). The two traces are triggered at points 180.degree. out of phase with each other, so that the positive half-wave of one waveform and the negative half-wave of the other waveform enclose an area shaped somewhat as the human eye.
The suitability of a transmission path for transmission of digital data is, to a large extent, dependent on the high-frequency rolloff of the transmission path, i.e., the manner in which the attenuation factor of the path increases as a function of the frequency of the signal to be transmitted. The severity of the high frequency rolloff of the transmission path manifests itself in the eye diagram as a distortion from its ideal form. Thus, instead of the CRT display showing an eye in which the peaks and troughs of the eyes are precisely half way between the intersections of the waveforms, the peak and trough between two successive intersections are closer, along the time axis, to the earlier intersection than to the later intersection. The use of eye diagrams is discussed at length in Chapter 27 of "Transmission Systems for Communications," Bell Telephone Laboratories, 1971, and Ingram, "Analysis and Design of Digital Transmission Systems," Computers and Digital Techniques, 1979, Vol, 2, No. 3.
One problem with the conventional method of providing an eye diagram is that of reliably triggering the horizontal sweep of the CRT. It is difficult to insure that the two traces of the CRT will trigger at precisely corresponding points on the sine wave.